The method of annihilation detection coincidence (ADC) is a very attractive detection technique in the field of nuclear imaging for medical purposes. This method makes use of he physical principle of electron-positron annihilation, which produces a pair of high-energy (511 Kev) photons. These photons propagate along a common line but in opposite directions. A two-headed gamma-ray camera is used to detect the locations where the photon-pair absorbed. An image reconstruction is accomplished by determining the liens along which the photons propagate from their point of production to their point of absorption. In order to reject scattered photons or stray photons not related to the pair being recorded, which if counted would produce a distorted or incorrect image, the energy of the photons and their timing (coincidence) is also measured. Since the method does not require a collimator, it is known as a collimator less method or a method of electronic collimation. Instruments designed according to this method have the advantages of improved sensitivity and of much reduced weight.
Imaging technologies based on Positron Emission Tomography (PET) that include multiple detectors and PET-like cameras having two detector-heads, require the use of detectors with high stopping power. The high stopping power is needed for efficient absorption of the high-energy photons. High stopping power is achieved by using thick detectors made of materials having a high atomic number, Z.
The traditional gamma-ray imaging technology presently used in nuclear medicine, including PET-like machines, uses Anger cameras, such as the type described in U.S. Pat. No. 3,011,057 to Anger. In this technology, the detectors are made of thick scintillators (such as sodium iodide NaI) combined with photo-multipliers. The more recently introduced semiconductor radiation detectors, such as those made of CdTe and CdZnTe, have the advantages of improved performance over scintillation detectors, in terms of improved energy and spatial resolution, count rate, stopping power and compactness. Accordingly such detectors have great potential to replace the traditional current technology of the Anger camera.
The idea of using a pixelated imaging-plane detector, consisting of multiple cells of semiconductor detector arrays is known in the art, as for instance described by H. H. Barrett, J. D. Eskin and H. B. Barber in their article "Charge transport in arrays of semiconductor gamma-ray detectors", published in Physical Review Letters, Vol. 75, pp. 156-159, 1995. Until recently, the very low yield associated with the growth of high quality semiconductor crystals, meant that the manufacturing process was costly and time consuming, which caused the above-mentioned idea to be unsuitable for implementation on a commercial basis.
Recent advances in crystal growth technology has improved the yield, enabling the production of relatively large modular pixelated detector arrays, which can be combined to form the complete imaging plane for gamma-ray and X-ray cameras. The current yield enables the economic production of pixelated detector arrays with typical module sizes of about 20.times.20 mm at the electrode surfaces, and several millimeters thick. Such relatively large modules of detectors have provided the commercial justification for the production of semiconductor gamma and X-ray cameras.
However, because the thickness of these detectors is limited to several millimeters, such cameras are suitable only or use in the energy range between X-ray and medium energy gamma-rays. In order to make such cameras suitable for proper operation with the method ADC, the thickness of the detectors must be increased to provide the high stopping power needed for high-energy photons (511 Kev). However, since such detectors would have higher volume, they would also have higher levels of grain boundaries, defects, traps and included non-uniformity in electric field, all of which degrade detector performance. The manufacturing yield thus goes down with the detector volume.
If the probability for producing a good module having a specific area of pixelated electrodes and of thickness d, is P, then the probability p of producing a good module having the same area, but of thickenss D, is given by:
p=p.sup.(D/d) (1)
This means that increasing the detector thickness for use with high-energy photons, while maintaining the same surface area, causes a significant reduction in the probability P of producing a good detector module. Alternatively, the same probability P of producing a good thick detector module would mean the reduction of the surface area of the detector modules, resulting in an area which is impractical for use.
There therefore exists a serious need for a detector module having a thickness with stopping power sufficient for use with high-energy photons, but which can be produced at a yield similar to that of thinner detector modules of similar detection area.
The disclosures of all publications and patents mentioned in this section, and in the other sections of the specification, and the disclosures of all documents cited in the above publications, are hereby incorporated by reference.